Signaling for wireless communications

ABSTRACT

A communication system, such as a wireless CDMA system, detects markers with fewer errors by having field units transmit the markers at different power levels (e.g., 9 dB for one marker and 11 dB for another marker). The difference in power levels of the markers allows the base station to identify the request markers using alternative criteria with a low probability of error, where the alternative criteria may include comparing the markers to respective energy level thresholds, monitoring occupancy of time slots, occupancy of mutually exclusive code channels, or combinations thereof. For example, in one particular embodiment, a request marker, which is generally a high priority marker, is transmitted with higher power, which improves the probability of detection and reduces the probability of false detection of the request marker.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/751,608, filed Jan. 28, 2013, which is a continuation of U.S. patentapplication Ser. No. 13/418,517, filed Mar. 13, 2012, now U.S. Pat. No.8,369,277, which issued on Feb. 5, 2013, which is a continuation of U.S.patent application Ser. No. 11/805,013, filed May 22, 2007, now U.S.Pat. No. 8,134,980, which issued on Mar. 13, 2012, which is acontinuation of U.S. patent application Ser. No. 10/171,080, filed Jun.12, 2002, now a U.S. Pat. No. 7,221,664, which issued on May 22, 2007,which claims the benefit of U.S. Provisional Patent Application Ser.Nos. 60/378,697, filed May 7, 2002, and U.S. Provisional PatentApplication Ser. No. 60/297,925, filed Jun. 13, 2001.

U.S. patent application Ser. No. 10/171,080 is also acontinuation-in-part of U.S. patent application Ser. No. 09/775,305,filed Feb. 1, 2001, now U.S. Pat. No. 7,079,523, which issued Jul. 18,2006, which claims the benefit of U.S. Provisional Patent ApplicationSer. No. 60/180,598, filed Feb. 7, 2000.

U.S. patent application Ser. No. 10/171,080 is also acontinuation-in-part of U.S. patent application Ser. No. 09/997,621,filed Nov. 29, 2001, now U.S. Pat. No. 6,678,260, issued Jan. 13, 2004,which is a continuation of U.S. patent application Ser. No. 09/730,376,filed Dec. 5, 2000, now U.S. Pat. No. 6,707,804, issued Mar. 16, 2004,which is a continuation of U.S. patent application Ser. No. 09/088,413,filed Jun. 1, 1998, now U.S. Pat. No. 6,222,832, issued Apr. 24, 2001,wherein each of the aforementioned applications are incorporated byreference as if fully set forth.

BACKGROUND

Increasing use of wireless telephones and personal computers has led toa corresponding increase in demand for advanced telecommunicationservices that were once thought practical only for specializedapplications. In the 1980s, wireless voice communications became widelyavailable through cellular telephone networks. Such services werethought at first to be for the exclusive province of businessmen becauseof expected high subscriber costs. The same was also true for access toremotely distributed computer networks, whereby until very recently,only business people and large institutions could afford the necessarycomputers and wireline access equipment.

As a result of the widespread availability of affordable newtechnologies, the general population now increasingly desires to havenot only wireline access to networks such as the Internet and privateintranets, but also wireless access as well. Wireless technology isparticularly useful to users of portable computers, laptop computers,hand-held personal digital assistants and the like who prefer access tosuch networks without being tethered to a telephone line.

There still is no widely available satisfactory solution for providinglow cost, high speed access to the Internet, private intranets, andother networks using the existing wireless infrastructure. This is mostlikely an artifact of several unfortunate circumstances. First, thetypical manner of providing high speed data service in the businessenvironment over a wireline network is not readily adaptable to thevoice grade service available in most homes or offices. For example,such standard high speed data services do not necessarily lendthemselves to efficient transmission over standard cellular wirelesshandsets because wireless networks were originally designed only toprovide voice services. As a result, present day digital wirelesscommunications systems are optimized for voice transmissions, althoughcertain schemes such as CDMA do provide some measure of asymmetricalbehavior for the accommodation of data transmissions. For example, thedata rate specified by the Telecommunication Industry Association (TIA)for IS-95 on the forward traffic channel is adjustable in incrementsfrom 1.2 kbps up to 9.6 kbps for so-called Rate Set 1, and incrementsfrom 1.8 kbps up to 14.4 kbps for Rate Set 2. On the reverse linktraffic channel, however, the data rate is fixed at 4.8 kbps.

At best, existing wireless systems therefore typically provide a radiochannel that can accommodate maximum data rate transfers of 14.4kilobits per second (kbps) over a forward link direction. Such a lowdata rate channel does not lend itself directly to transmitting data atrates of 28.8 or even 56.6 kbps that are now commonly available usinginexpensive wireline modems, not to mention even higher rates such asthe 128 kbps that are available with Integrated Services Digital Network(ISDN) type equipment. Data rates at these levels are rapidly becomingthe minimum acceptable rates for activities such as browsing web pages.

Although wireline networks were known at the time when cellular systemswere initially developed, for the most part, there was no provision madefor such wireless systems to provide higher speed ISDN- or ADSL-gradedata services over cellular network topologies.

In most wireless systems, there are many more potential users than radiochannel resources. Some type of demand-based multiple access system istherefore required.

Whether the multiple access is provided by the traditional FrequencyDivision Multiple Access (FDMA) using analog modulation on a group ofradio frequency carrier signals, or by schemes that permit sharing of aradio carrier frequency using Time Division Multiple Access (TDMA), orCode Division Multiple Access (CDMA), the nature of the radio spectrumis such that it is expected to be shared. This is quite dissimilar tothe traditional environment supporting data transmissions in which thewireline medium is relatively inexpensive and is not typically intendedto be shared.

Other factors to consider in the design of a wireless system are thecharacteristics of the data itself. For example, consider that access toweb pages generally is burst-oriented, with asymmetrical data ratetransmission requirements in a reverse and forward direction. In acommon application, a user of a remote client computer first specifiesthe address of a web page to a browser program. The browser program thensends the web page address data, which is usually 100 bytes or less inlength, over the network to a server computer. The server computer thenresponds with the content of the requested web page, which may includeanywhere from 10 kilobytes to several megabytes of text, image, audio,or even video data. The user thereafter may spend several seconds oreven several minutes reading the content of the page before downloadinganother web page.

In an office environment, the nature of most employees' computer workhabits is typically to check a few web pages and then to do somethingelse for an extended period of time, such as accessing locally storeddata or even terminating use of the computer altogether. Therefore, eventhough such users may remain connected to the Internet or privateintranet continuously during an entire day, actual use of the high speeddata link is usually quite sporadic.

If wireless data transfer services supporting Internet connectivity areto coexist with wireless voice communication, it is becomingincreasingly important to optimize the use of available resources inwireless CDMA systems. Frequency re-use and dynamic traffic channelallocation address some aspects of increasing the efficiency of highperformance wireless CDMA communication systems, but there is still aneed for more efficient utilization of available resources.

SUMMARY OF THE INVENTION

In one application, a transmission of a marker in a time slot over onechannel indicates a request by the corresponding field unit to goactive. That is, transmission of a marker in an assigned time slotindicates that the field unit is requesting that reverse link trafficchannels be assigned to the user for transmitting a data payload fromthe field unit to the base station. This presumes that the field unit ispresently in the standby mode. Alternatively, a field unit transmits amarker over a second channel of the pair of reverse link channels toindicate that the field unit is not requesting to be placed in theactive mode. For example, the field unit does not want to transmit dataon a reverse link channel. Rather, the field unit requests to remaininactive but synchronized with the base station so that the field unitcan immediately go active again at any moment.

In either case, a wireless communications system employing theprinciples of the present invention may improve performance of detectingthe markers by having the field units transmit the markers at differentpower levels (e.g., 9 dB for one marker and 11 dB for the other marker),which can improve system performance. The difference in power levels ofthe markers allows the base station to identify the request markersusing alternative criteria with a low probability of error, where thealternative criteria may include comparing the markers to respectiveenergy level thresholds, monitoring occupancy of time slots, occupancyof mutually exclusive code channels, or combinations thereof. Forexample, in one particular embodiment, a request marker, which isgenerally a high priority marker, is transmitted with higher power,which improves the probability of detection and reduces the probabilityof false detection of the request marker.

In one particular CDMA system application, the field unit provides aHeartbeat (HB) channel using a first code in a reverse link to the basestation and a heartbeat-with-request (HB/RQST) channel using a secondcode in the reverse link. In this CDMA application, according to theprinciples of the present invention, the field unit may transmit the HBand HB/RQST channels with a difference in power levels, preferablygiving the HB/RQST the higher power since it is a higher prioritysignal.

The teachings of the present invention support I-CDMA and 1xEV-DVsystems, but are general enough to support systems employing variousother communications protocols used in wired or wireless communicationssystems. Code Division Multiple Access (CDMA) systems, such as IS-2000,and Orthogonal Frequency Division Multiplexing (OFDM) systems, such asIEEE 802.11a wireless local area network (LAN), may employ an embodimentof the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a schematic diagram of a communications system in which in anembodiment of the present invention may be deployed;

FIG. 2 is a schematic diagram of a subsystem employed by a base stationin the communications system of FIG. 1 used to determine whether areverse link signal includes an indication for a request to changecommunications states based on an energy level in the signal;

FIG. 3A is a signal diagram of a 1xEV-DV signal with a first markerindicating ‘control hold’ and a second marker indicating a ‘request togo active’;

FIG. 3B is a signal diagram of a code division multiple access (CDMA)set of code channels having a marker in an assigned time slot thatindicates that the field unit is requesting a change in communicationsstates;

FIG. 3C is a signal diagram of an alternative embodiment of a reverselink signal having the indications; and

FIG. 4 is a plot of signal-to-noise ratio versus probability ofdetection that may be used in determining energy levels of theindications in the signals of FIGS. 3A-3C.

DETAILED DESCRIPTION OF THE INVENTION

A description of the preferred embodiments of the invention follows.

In a wireless communications system, an embodiment of the presentinvention applies to the power that is transmitted from a handset (orthe target received power at a base terminal station (BTS>> for aHeartbeat signal (HB) versus a Heartbeat-with-Request signal (HBR,HB/RQST, or just the “request” signal). The HB and HB/RQST signals maybe transmitted on a maintenance channel, which, as disclosed in U.S.Ser. No. 09/775,305, is a single code channel (out of many) on a reverselink of a CDMA communications system. The maintenance channel is timeslotted and different users are assigned different slots.

A field unit in that wireless communications system sends a heartbeatsignal to maintain timing and/or power control as well as an indicationof presence to the BTS. When a terminal needs an assigned reverse linkchannel, the terminal then transmits at least one request signal. Thesignal(s) may be modulated messages or simply coded pilot signals withno “bits”.

The requirements for probability of detection and probability of falsedetection for these channels are quite different. For instance, thedetection requirement for HB is relatively low. It may only need to bedetected at a rate that is fast enough to track the slew of the timingof the code channel due to physical movement of multi-path structurechange resulting from Doppler in the channel. The power control in thiscase continues to work independent of detection or lack of detection.

For instance, if the signal is not “detected” because the received poweris not above a predetermined threshold but the correlation is aligned,the power command indicates that the power was too low and that theterminal should “power up”. One requirement, in this particularembodiment, is that the detection occur often enough to allow thedetector to be aligned in time to the received signal.

On the other hand, the probability of detection for the request signalis preferably very high as a request signal is considered a highpriority signal since a request is an urgent event. Therefore, therequest signal may be sent with higher power, and the threshold at theBTS may be set differently. This results in a better probability ofdetection as well as a low probability of false detection.

Thus, according to the principles of the present invention, a differentprobability of detection and probability of false detection for theheartbeat signal, request signal, or any other signaled message can beemployed.

Based on the signal type, an access terminal can transmit the signalswith different powers. Various criteria can be used by the BTS fordetecting indications of a request sent on the signals. For example, intime slotted channels or mutually exclusive code channels, some slotsare occupied when a request is being made versus when a request is notbeing made. In such a case, either a higher power, presence, or both maybe used as detection criteria.

FIG. 1 is a diagram of an example communications system 100, similar tothe system described above, employing an embodiment of the presentinvention. A base station 25 with antenna tower 23 maintains wirelesscommunications links with each of a plurality of field units 42 a, 42 b,42 c (collectively, field units 42) as shown. Such wireless links areestablished based upon assignment of resources on a forward link 70 anda reverse link 65 between the base station 25 and field units 42. Eachlink 65 or 70 is typically made up of several logical reverse linkchannels 55 and several logical forward link channels 60, respectively.

As shown, the communications system 100 supports wireless communicationsbetween an interface 50 and a network 20. Typically, the network 20 is aPublic Switched Telephone Network (PSTN) or computer network, such asthe Internet, internet, or intranet. The interface 50 is preferablycoupled to a digital processing device, such as a portable computer 12,sometimes referred to as an access unit, to provide wireless access tothe network 20. Consequently, the portable computer 12 has access to thenetwork 20 based on communications over a combination of both hard-wiredand wireless data links.

In a preferred embodiment, the forward link channels 60 and reverse linkchannels 55 are defined in the communications system 100 as CodeDivision Multiple Access (CDMA) channels. That is, each CDMA channel ispreferably defined by encoding and transmitting data over the channelwith an augmented pseudo random noise (PN) code sequence. The PN codeddata is then modulated onto a radio frequency carrier. This enables areceiver to decipher one CDMA channel from another knowing only theparticular augmented PN code assigned for a given channel. In accordancewith an embodiment, each channel may occupy a 1.25 MHZ band consistentwith the IS-95 CDMA standard and 1xEV-DV standard and is capable oftransmitting at 38.4 kbps.

A forward link 70 includes at least four logical forward link channels60. As shown, this includes a Pilot Channel 60PL, Link QualityManagement (LQM) channel 60L, paging channel 60PG, and multiple trafficchannels 60T.

A reverse link 65 includes at least five logical reverse link channels55. As shown, this includes a heartbeat standby channel 55HS, heartbeatrequest active channel 55HRA, access channel 55A and multiple trafficchannels 55T. Generally, the reverse link channels 55 are similar to theforward link channels 60 except that each reverse link traffic channel60T can support variable data rates from 2.4 kbps to a maximum of 160kbps.

Data transmitted between the base station 25 and field unit 42 atypically consist of encoded digital information, such as web page data.Based on the allocation of multiple traffic channels in the reverse link65 or forward link 70, higher data transfer rates can be achieved in aparticular link between the base station 25 and field unit 42 a.However, since multiple field units 42 compete for bandwidth allocation,a field unit 42 a may have to wait until resources are free to beassigned traffic channels to transmit a data payload.

Before discussing an example detector system (FIG. 2) that can be usedto distinguish a heartbeat from a heartbeat-with-request signal, a briefdiscussion of example signals will be discussed in reference to FIGS.3A-3C.

In FIG. 3A, a 1xEV-DV signal 160 that may be transmitted by the fieldunit is shown having three distinct states: a ‘control hold’ state 165,a ‘request to go active’ state 170, and a data traffic state 175. In the‘control hold’ state 165, the signal 160 does not include a ‘request togo active’ indication. In other words, the signal 160 remains in an‘idle’ or ‘control hold’ state, which indicates that the field unit 42 ais not requesting traffic channels. The ‘request to go active’ state 170is an indication that the field unit is requesting to transmit data on atraffic channel over a reverse link to the BTS 25. In the traffic state175, traffic data is transmitted by the field unit to the BTS. Followingtransmission of the traffic data over the reverse link, the signal 160reverts back to the ‘control hold’ state 165 following a transmission ofa ‘data transmission complete’ state (not shown).

Although shown as a single signal 160, it should be understood that thesignal may be multiple signals, optionally coded with orthogonal ornon-orthogonal codes into mutually exclusive channels. For example, the‘control hold’ state 165 may be transmitted on a different channel fromthe ‘request to go active’ state 170. Similarly, the traffic datatransmitted in a traffic state 175 may be on a separate channel from theother two states 165, 170. An example of multiple channel is discussedin reference to FIGS. 3B and 3C.

FIG. 3B is an example of an Internet code division multiple access(I-CDMA) signaling diagram that has assigned time slots for users 1, 2,3, . . . , N repeating in epoch i 177 a, epoch i+1 177 b, and so forth.The channels are composed of the heartbeat channel 55H, request channel55R, and traffic channels 55T. Each of these channels has an associatedcode C1, C2, C3, C4, . . . “CN, which allow signals to be transmitted onmutually exclusive code channels. Both the transmitting and receivingsystems process the information in the channels by using the codes toseparate the information respectively included therein in a typical CDMAmanner.

In the example shown, users 1, 2, 4, 5, 6, . . . , N are requesting toremain in an idle state, indicated by the presence of a signal 180 inthe heartbeat channel 55H. User 3, however, is requesting to transmitdata over a reverse link based on a signal 185 in the request channel55R in the first epoch 177 a. In the second epoch 177 b, user 3 beginsto transmit traffic data 190 in an associated traffic channel using codeC5.

FIG. 3C is a more detailed signal diagram of the 1xEV-DV signal of FIG.3A that is used to indicate a ‘request to go active’ to the base station25 from the field unit 42 a. In this embodiment, the 1xEV-DV signal iscomposed of multiple signals on different logical channels: a heartbeatchannel 55H and a request channel 55R. The heartbeat channel 55Hprovides continuous timing and other information (e.g., power level,synchronization, etc.) from the field unit 42 a to the base station 25.The field unit 42 a uses the request channel 55R to make a request(e.g., digital “1”) of the base station 25 to request a traffic channelon the reverse link 65 for transmitting data.

Sampling time periods 195 a, 195 b, . . . , 195 f (collectively 195)denoted by arrows indicate times or intervals at which the BTS 25samples the time slots of the request signal 55R and, optionally, theheartbeat channel 55H to determine whether a request for a trafficchannel is being made. It should be understood that the sampling mayoccur over the entire time slot or a subset thereof. Also, the heartbeatchannel 55H and request channel 55R use mutually exclusive codes, inthis particular embodiment, so the sampling is performed on theirmutually exclusive code channels 55H, 55R in all or a subset of timeslots. In one particular embodiment, the base station 25 samplesmutually exclusive code channels 55H, 55R in time slots designated forrequest indications, such as in time slots at sampling times 195 b, 195d, and 195 f. During these time slots, the heartbeat channel 55H is“inactive,” but the request channel 55R is “active”.

As discussed above, the signals in the “active” request time slots maybe modulated messages or simply coded pilot signals with no “bits”.Thus, detection may be based solely on the respective energy levels ofthe heartbeat and heartbeat-with-request signals in respective timeslots over a given time interval or spanning several time intervals. Inone particular embodiment, the ‘control hold’ state 165 indication has afirst energy level, and the ‘request to go active’ state 170 has asecond energy level.

In this particular embodiment, distinguishing the two states may be amatter of measuring energy levels of the signals(s) and (i) comparingthe energy levels against at least one threshold or (ii) determiningthat a request is present, optionally in a mutually exclusive codechannel in time slots when the heartbeat signal is at a logical zero.The different energy levels of the indications may be provided by theduty cycle of the signals, frequency of the signals, power of thesignals, signaling structure, and so forth.

To understand how the energy levels of the signals can be used toimprove system performance, one can refer to FIG. 4, which provides achart for selecting signaling requirements based on the followingparameters or factors: (i) probability of detection, P(d)(x-axis), (ii)signal-to-noise ratio in decibels (y-axis), and (iii) probability offalse detection, P(fd) (curves in the chart). This chart shows arequired signal-to-noise ratio at the input terminals of alinear-rectifier detector as a function of probability of detection fora single pulse, with the false-alarm probability P(fd) as a parameter,calculated for a non-fluctuating signal. It should be understood thatalternative parameters or factors may be used to establish or define thetransmitted power levels of the indications.

At the circled point 200, the signal-to-noise ratio is 3 dB, P(d)=20%,and P(fd)=1%. To increase the probability of detection for the sameprobability of false detection, one simply needs to slide the circledpoint 200 upward along the same probability of false detection curve,which suggests that an increase in the signal-to-noise ratio is used toimprove system performance and, thus, improving the likelihood that therequest signal will be detected quickly.

Before providing an example model and discussion regarding exampleHeartbeat standby 55HS and Heartbeat Request Active 55HRA energy levelsfor the example communications system 100 (FIG. 1), a brief discussionof a processor and detector that may be used in the system is nowprovided.

FIG. 2 is a schematic diagram of a request detection processor 110 usedto determine whether the field unit 42 a has requested to send data tothe BTS 25. The receiver Rx 35 receives signals 55, which includes themaintenance channel 55N, traffic channels 55T, access channel 55A,heartbeat standby channel 55HS, and heartbeat request active channel55HRA. The reverse link channels 55 are processed such that a heartbeatchannel processor 112 receives the heartbeat standby channel 55HS and arequest channel processor 114 receives the Heartbeat Request Activechannel 55HRA.

The heartbeat channel processor 112 and request channel processor 114include the same processing elements, in this particular embodiment, soa discussion of just the heartbeat channel processor 112 will beprovided for brevity.

The heartbeat channel processor 112 receives the heartbeat standbychannel 55HS. A correlator 115 uses a despreader 120 to despread theheartbeat standby channel 55HS. An integrator 125 is used to coherentlycombine the heartbeat signal. By coherently combining the signal, anintegration of I, Q and its phase causes the phase of the signal to beremoved and output the power of the signal.

Following the correlator 115, a rectifier 130 (i.e., absolute value ofthe signal squared) rectifies the power of the signal, which is thenintegrated by a second integrator 135 to calculate the energy of thereceived heartbeat signal. The second integrator 135 providesnon-coherent combination of the signal, which is calculated over shorttime intervals. The non-coherent integration provides just magnitudes ifthe terminal is moving too fast, thus causing a cross-over of the1S0-degree phase point, which can cause ambiguities in determining theenergy of the signal in the absence of the non-coherent combination.

The output from the heartbeat channel processor 112 is a heartbeatenergy level, and the output from the request channel processor 114 is arequest energy level. Each of these energy levels, in this particularembodiment, is fed to a hypothesis detector 140, which determineswhether a heartbeat signal, request signal, or neither signal is in thereverse link channels 55 received by the base station 25.

To determine which signal(s) is/are present, the hypothesis detector 140includes logical functions. For example, in this particular embodiment,the hypothesis detector 140 compares a first energy level thresholdagainst the first energy level (i.e., heartbeat energy level) andcompares a second energy level threshold against the second energy level(i.e., request energy level).

An example energy level threshold to compare the heartbeat energy levelagainst is 9 dB and the request energy level threshold is 11 dB. Theenergy level thresholds may be dynamically selected, predetermined, orapplied in another manner, such as based on a transmitted power level,which may be reported by the field unit to the base station over theheartbeat channel 55H, for instance. In the case of the energy levelcalculation and comparison, the first and second energy levels may bedependent on occupancy of time slots in the signaling channel(s) used bythe signal 55, so the energy level thresholds can be based on anexpected or specified number of “1” bits used to indicate a ‘request togo active’ or to indicate a request to remain in idle mode.

The output of the hypothesis detector 140 may be used to change thestate of the communications system. For example, if the hypothesisdetector 140 determines that a ‘request to go active’ (i.e., send a datatransmission on the reverse link) is being made by the field unit, thenthe hypothesis detector outputs a signal to a processor (not shown inthe BTS 25) that is responsible for providing the portable computer 12with a traffic channel 55T. In one embodiment, the BTS 25 allocates thetraffic channel 55T if the detected energy level of the signal isdetermined to be above the second energy level threshold. Alternatively,the BTS allocates the traffic channel 55T if the hypothesis detector 140determines that the detected energy level is below the second energylevel threshold.

As described in reference to FIG. 3C, the heartbeat channel processor112, request channel processor 114, and hypothesis detector 140 may beconfigured or designed in a manner that monitors an occupancy of timeslots used to indicate the request to change communications states. Inone embodiment, the detecting includes monitoring occupancy of mutuallyexclusive code channels, such as shown in FIGS. 3B and 3C.

A feedback loop (not shown) may be employed to cause the heartbeatchannel processor 112 and request channel processor 114 to be“adaptive”. For example, based on the received energy level of theheartbeat channel 55H, the integration time of the integrators 125, 135may be adjusted, and the energy level thresholds used by the hypothesisdetector 140 for comparison of the energy levels of the heartbeat andrequest signals may also be adjusted by the feedback loop. Such afeedback loop may use a command or message to transfer informationbetween the BTS and field unit that includes information regarding thepower levels of the heartbeat or heartbeat-with-request signalstransmitted by the field unit.

As discussed above, the first communications state may be a standbycommunications state and the second communications state may be apayload communications state. In other systems or even the same system,the communications states may refer to other communications states, suchas a request to change base stations, power control signaling, and soforth. The use of different energy levels in signaling as describedherein is applicable to wireless, wired, or optical communicationssystems. In either case, the communications states may be used in voiceor data communications systems.

As also discussed above, the second energy level may be based on atarget probability of detection, false detection, or combination of bothas discussed in reference to FIG. 4. In other words, the field unit maytransmit the request signal at a given power level or a given number ofpulses per given time period to achieve a corresponding signal-to-noiseratio for a given target probability of detection, false detection, orboth as discussed in reference to FIG. 4.

An analysis may be used to set the transmission power or number oftransmitted indications, or a feedback mechanism may be employed in thecommunications system for causing the field unit to change its behaviorso as to have the received energy levels of the indications achieve apredetermined signal-to-noise ratio, thus providing the desiredprobability of detection and false detection parameters.

Simulation:

A simulation is now presented that discusses the trades effecting theprobabilities of detection and false detection for the heartbeat (HB)and heartbeat-with-request (HB/RQST) channels. Recommended SNR targetsfor HB and HB/RQST channels are provided. Further, an analyticalcalculation is made to determine a recommended target E/lo for anacceptable probability of detection and probability of false detection.

To allow readers to baseline the simulation relative to IS-2000 powercontrol, the reader should be aware that the simulation employs thefollowing parameters:

800 Hz closed loop power control;

SNR of the i'th user is calculated asSNR(i)=P(i)−P_interference+processing Gain+Er, where P_interference(i)is a total received interference for the i'th user and calculated as Pinterference(i)=20*log 10(10)̂Σ_(j≠i) (10̂P(j/20)+10̂(P_(TH)/20)), whereP(i) is the power received from the i'th user and PTH is thermal noisefloor and is arbitrarily set to 120 dBm;

processing gain is 10 log 64;

fading model is Jakes;

Er=a Normal distributed random variable with 1 sigma=0.67 dB error inSNR estimation at the BTS; and

power control bit (PCB) errors=3%.

In this particular simulation, a choice of a target SNR for the HBchannel was chosen first. Based on a 9 dB E/lo, where E is the entireenergy in the heartbeat message and a 95% probability of detection witha 0.1% false detection rate in Additive White Gaussian Noise (AWG) isachieved (see Viterbi, A., CDMA: Principles of Spread SpectrumCommunication, Addison Wesley, 1995, p 113.)

Increasing the probability of detection to 99% yields significantlyhigher false detection rate of 1% in AWGN. This false detection rate isof interest as it should be low enough that non-detection will occur fora relatively long duration when the terminal has dropped thecommunications link with the base station.

Typically, the duration is defined by a timer having a duration of 500ms to 2 sec or 25 to 100 sequential non-detections. For reference, in asingle path fading environment with 9 db E/lo, a 90% detectionprobability and a 1% false detection rate is predicted by theory. Forthis case, the details associated with the detection probability in thefading environment are considered in the following discussion.

Detection of the heartbeat signal with 50 Hz power control versus fieldunit velocity is now considered. The simulation is based on a full ratemodel in which modifications were made such that the power control (PC)rate is 50 Hz and standby terminals are time slotted, non-overlapping.

While the speed of the terminal is irrelevant above about 2 mph,closed-loop power control is seen as useful in allowing the fading tovary around the mean path loss. It is noted that the results arerelatively insensitive to a Power Control Bit (PCB) error rate up toabout 40%. Beyond that, the system performed poorly, demonstrating thatsome form of closed-loop control is needed to maintain the mean pathloss. It is therefore useful that some form of closed-loop power controlbe performed to bring the transmitter (Tx) power of the field unit tothe average appropriate for the field unit to achieve the mean path lossto the base station.

Simulation using the above parameters shows that if the base stationdetects the ‘request to go active’ indication 2 dB below the target SNR(as defined above), then the average time of detection is about 16 ms,with standard deviation at about 14 ms. From the simulation, to achievea low latency in HB/RQST detection, the following equation has beendetermined:

Target_SNR(RQST)=Target_SNR(HB)+2 dB  (1)

Based on the required detecting/false detection rates m AWGN, aTarget_SNR of 9 dB for the Heartbeat message and 11 dB for theheartbeat-with-request (HB/RQST) message were chosen. These parametersyield a 15 ms detection latency on average at 20 mph with lowprobability of false detection.

In terms of false assignment probability, while the false detection rateis not explicitly calculated in the simulation, a pessimistic bound isgiven as follows:

$\begin{matrix}{{{{Pfd}({RQST})} = {{\left( {1 - {{Pd}({HB})}} \right)*{{Pfd}({HB})}} = {{5\%*0.1\%} = {{5\; E} - 5}}}},} & (2)\end{matrix}$

where Pfd is the probability of false detection and Pd is theprobability of detection.

The above equation and result is a product of two conditions: (i) notdetecting the existence of a HB while it is present and (ii) falselydetecting a HB when it is not present. This is a pessimistic bound sincethe additional 2 dB of transmitted power for HB/RQST versus HB is notincluded in the analysis.

At a HB rate of 50 Hz, this would yield a false assignment for onestandby user every 400 seconds on average. For N heartbeat users, theprobability is linear since the events are independent. Therefore, for afully loaded standby user population of 96 for a particular basestation, the average false assignment rate is expected to be roughly oneevery four seconds.

A condition of a false assignment can be recovered from relativelyquickly as the false assignment can be detected quickly. When a falseassignment occurs, three conditions are typically present. First, notraffic shows up on the assigned reverse channel. Second, theheartbeat-with-request signal is not present. If a missed channelassignment has occurred, the HB/RQST continues to be present. Third, theheartbeat message will likely be present. The probability of notdetecting this condition in one frame is Pfd(RQST)=5E-3%. This should bedetected within one or two frames before the channel can be re-assignedto a legitimate user. If it is assumed that detection takes two frames,the reverse capacity would be reduced by no more than 1% and is likelyless as the probability of false detection for the HB/RQST is targetedat 11 dB E/lo.

For a signal with no offset between the Target_SNR and the detectionthreshold, the delay to detection is 35 ms on the average between thesimulation with a remote subscriber unit moving at 1 mph and at 20 mph.For the heartbeat-with-request (HB/RQST) signal, the average delay todetection is less than 20 ms, with a detection threshold of 2 dB belowthe target SNR of 11 dB. This is possible since the transmit (Tx) poweris increased by 2 dB for the HBIRQST relative to the HB signal.

The simulation shows that the minimum average, given 96 users in a 20 mspower control (PC) period is close to 10 ms. The delay is expected to bebetter than 75 ms 99% of the time.

The simulation also shows that the addition of 2 dB of additionaltransmit power for the HB/RQST message increases the detectionprobability and reduces the detection latency to 15 ms on average. Anestimate of the co-channel total interference of a fully loadedmaintenance channel is between 6 dB less than an IS-2000 fundamentalchannel (9600 bps Reverse Traffic Channel (R-TCH), 9600 bps ReverseDedicated Control Channel (R-DCCH>>.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A field unit comprising: at least one processorconfigured to transmit a first signal having one of a plurality offormats; wherein at least two of the formats have a different poweroffset; wherein the first signal is transmitted in response to the fieldunit not having an allocation of traffic data resources to maintaintiming and power control and as an indication of presence within anetwork; on a condition that the field unit requires an allocation oftraffic data resources, the at least one processor configured totransmit the first signal with a format of the plurality of formatsassociated with a request; in response to the request, the at least oneprocessor configured to receive an allocation of traffic data resourcesand transmit reverse link data in response to the allocation in a timeinterval including at least one time slot; and the at least oneprocessor configured to receive power commands and adjust thetransmission power level of the first signal in response to the powercommands.
 2. The field unit of claim 1 wherein the power offsets are infixed dB values.
 3. The field unit of claim 1 wherein the first signalwith the format associated with the request is derived from at least anorthogonal code.
 4. The field unit of claim 1 wherein the first signalwith the format associated with the request does not have a modulatedbit.
 5. The field unit of claim 1 wherein the at least one processor isonly configured to transmit the first signal on a condition that thereverse link timing is maintained with a wireless network.
 6. A fieldunit comprising: at least one processor configured to maintain aconnection with a network during packet transfer by: in response tohaving data to transmit and having no assigned reverse link resources,transmitting a request for resources in a time interval including atleast one slot, and in response to not having data to transmit andhaving no assigned resources, to not transmit the request in the timeinterval; in response to transmitting the request for resources, the atleast one processor configured to receive an assignment of reverse linkresources and to transmit reverse link data in response to theassignment of reverse link resources in a second time interval includingat least one time slot to maintain timing and power control and as anindication of presence within a network; and in response to receivedpower commands, the at least one processor configured to adjust atransmission power level of the request or the reverse link data inaccordance with the power commands.
 7. The field unit of claim 6 whereinthe request is transmitted as a signal not including a modulated bit. 8.The field unit of claim 6 wherein the at least one processor is furtherconfigured to transmit in a third time interval other than the first andsecond time interval, a signal having a power level offset from atransmission power level of the request.
 9. The field unit of claim 6wherein the power offset is a fixed dB value.
 10. The field unit ofclaim 6 wherein the request is transmitted as a signal derived from anorthogonal code.
 11. The field unit of claim 6 wherein the at least oneprocessor is only configured to transmit the request on a condition thatthe reverse link timing is maintained with a wireless network.
 12. Abase station comprising: at least one processor configured to maintainconnections with subscriber units transmitting packet data by receivingtransmissions from the subscriber unit in time intervals when thesubscriber units are not transmitting packet data, wherein the receivedtransmissions are in a plurality of formats and at least one of theformats includes an indication that a subscriber unit desires totransmit packet data to maintain timing and power control and as anindication of presence within a network; and the at least one processorconfigured to determine whether a subscriber unit is requesting totransmit data by detecting an energy level associated with the timeinterval.
 13. The base station of claim 12 wherein the processor isfurther configured to transmit power commands to the subscriber units toadjust power levels of the transmissions in the time intervals when thesubscriber units are not transmitting packet data.
 14. The base stationof claim 12 wherein the received transmissions are derived from at leastone orthogonal code.